The invention relates to a process for preparing an amorphous ultrahard material based on boron nitride, starting from hexagonal or turbostratic boron nitride, which is compressed at pressures of at least 70 kbar or above and heated to temperatures of at least 1650.degree. C. or above, until a boron nitride melt is obtained, and the boron nitride melt formed is quenched by shutting off the heat supply, and the quenched boron nitride melt is then relieved of the pressure.
A congeneric process for preparing ultrahard boron nitride, which is amorphous with respect to electron beams and X-rays, and which is designated as aBN-2, having a hardness sufficient to scratch diamond, is disclosed by WO 90/02704 and by the German Patent 38 30 840.
Industrial techniques for preparing single crystals or sintered bodies of cubic boron nitride make effective use of processes, in which the starting material of hexagonal or pyrolytic boron nitride, with the addition of crystallisation catalysts and solvent catalysts, is converted, at a high rate of conversion, into boron nitride having a cubic or wurtzite structure. Suitable references in this context include U.S. Pat. Nos. 4361 543, 3,192,015, 3,233,988 and 4,188,194.
A decisive factor in this process is the influence of the catalysts on the new growth of a crystalline boron nitride having a greater hardness, as well as a high temperature stability, than the starting material.
Three crystalline structures of boron nitride are known: a soft hexagonal form, also designated as white graphite, the hexagonal boron nitride hBN; a hard hexagonal wurtzite-type form similar to the hexagonal diamond structure, the so-called wurtzite-type boron nitride wBN, as well as a cubic zinc blende form, whose structure is similar to that of cubic diamond, and which is designated as cubic boron nitride cBN. In the accompanying drawing, FIG. 1 shows the crystal lattice and the crystal lattice layers of the hexagonal boron nitride hBN, FIG. 2 shows the wurtzite-type crystal lattice structure of the wBN, and FIG. 3 shows the crystal lattice structure of the cubic boron nitride cBN.
All three modifications of boron nitride are used in industry as powders or sintered solids, on the one hand as raw materials for syntheses, on the other hand as abrasive grain for use in tools. Sintered solids of cBN are also used as heat sinks in microelectronics.
With regard to the hard boron nitride phases which are processed into compact sintered solids, a distinction is made between types with self-binding of the grains or binding with the aid of a separate binder between the boron nitride grains. Both types have been used hitherto for producing nonporous sintered compacts. Owing to the admixture of less hard substances, the last-mentioned type, however, is less suitable for use in tools.
The direct conversion process from hBN to cBN or wBN is also feasible, see for example the Japanese Patents 49/27518, 49/30357, 49/22925, and the U.S. Pat. No. 3,852,078, inter alia, which operates at pressures in a range above 50 kbar and at temperatures from 1100.degree. to 3000.degree. C.
In addition to the crystalline forms of boron nitride, there are at least two amorphous forms of different hardness. The first amorphous form is designated as aBN-1 or pBN, which stands for pyrolytic boron nitride. The amorphous boron nitride called aBN-1 or pBN is soft like hBN, see for example Japanese Published Specification 62-263962, and is produced without pressure in a CVD process. The second amorphous form of boron nitride is designated as aBN-2 and is notable for extremely high hardness, so that it is able to scratch, drill and mill diamond even in the (111) direction, as well as excelling by high temperature stability up to over 1450.degree. C. Amorphous boron nitride aBN-2 of such ultrahardness is first described in the German Patent 3830 840. With regard to the aBN-1 (pBN) there is some dispute, see U.S. Pat. No. 4,188,194, whether there is not after all some order in the crystal structure, since the boron and nitrogen atoms, similarly to the graphite lattice of hBN, are preferentially linked on certain planes. These planes, however, are not crosslinked periodically in three dimensions.
All the industrially significant processes for preparing the hard boron nitride phases take place at high pressures and temperatures, since the action of a coercive external force and a supply of thermal energy are required to induce positional interchange of the atoms in the lattice of the starting materials and thus to obtain a new, denser structure. If substances with specific catalytic effects are introduced into the boron nitride, however, the conversion into a particular structural form is predetermined if sufficiently high pressures and temperatures are employed.